This invention relates to light emitting diodes and, more particularly, to a system method for driving multiple light emitting diodes so as to reduce the wavelength variation between them.
Light emitting diodes (LEDs) are generally manufactured in batches using standard semiconductor fabrication techniques. A single semiconductor wafer will typically yield multiple LEDs. Although the fabrication process can be controlled to obtain LEDs that emit light at a specific color, there are generally significant variations in the output wavelengths of the LEDs when they are driven with a common drive current.
For example, a batch of LEDs can be designed to emit green light, however, one LED could emit light at 500 nm, while another LED could emit light at 506 nm. Even with LEDs that originate from a single wafer, the output wavelengths can vary significantly. LEDs that originate from different wafers can exhibit even greater wavelength variations.
In some LED applications, wavelength variations between the LEDs can be undesirable. For example, automobile manufacturers often create a vehicle""s unique identity, in part, through the use of xe2x80x9cthemexe2x80x9d wavelength for the interior trim and illumination. This illumination can include backlighting of switches, instrument cluster backlighting, and general or specific illumination applications. If LEDs are used, the theme wavelength requirements generally dictate that their output light fall within a narrow range of wavelengths.
Because the output wavelengths of individual LEDs can vary by more than the amount of deviation allowable for certain applications, in many cases not ail of, the LEDs from a single semiconductor wafer can be used. Because some of the LEDs from a single wafer may be rejected for a particular application, the costs associated with utilizing LEDs is increased.
In an exemplary embodiment of the invention, a method of driving a plurality of LEDs comprises the steps of: determining an emission wavelength of each of the plurality of LEDs at a common drive current; choosing a respective operational drive current for each of the plurality of LEDs such that wavelength variations between the plurality of LEDs, when driven at their respective operational drive currents, is less than wavelength variations between the plurality of LEDs when driven at the common drive current; and driving the plurality of LEDs with the respective operational drive currents.
The invention also provides a lighting system comprising: a plurality of LEDs that, when driven with a common drive current, collectively emit light with initial wavelength variations; and a drive circuit for driving the plurality of LEDs with respective operational drive currents, such that the plurality of LEDs collectively emit light with operational wavelength variations that are less than the initial wavelength variations.
The system and method of the invention minimizes wavelength variation between LEDs, thereby allowing the use of more, and in some cases all, of the LEDs that are fabricated from a single semiconductor wafer, or multiple semiconductor wafers. This reduces the costs associated with utilizing LEDs in lighting applications that require a narrow range of output wavelengths.